In general, a photomask (having a part for transmitting light and another part for shielding light) is mostly used in photolithography, especially as applied to semiconductor device fabricating processes. However, the general photomask, which has such a light transmission pattern and a light shielding pattern for selective exposure, has a limitation in improving its resolution because of diffraction coming from increased pattern packing density. Therefore, research for improving the resolution in the phase shift mask has been under way in many fields.
The phase shifting mask combines light transmissive regions (which transmit light without shifting it in phase) and light shifting transmissive regions (which transmit light phase-shifted by 180.degree.). As light passes through an opening such as a light transmitting region it diffracts, which leads to unwanted constructive interference in areas between light transmitting regions. To reduce this unwanted constructive interference, the light shifting regions are provided to create destructive interference by way of 180.degree.-phase-shifted light. Such phase shift masks have improved resolution.
For fabrication of an accurate phase shift mask, defect inspection and defect correction must be conducted. The defects can occur during a mask pattern fabricating process or a photolithography process for a substrate or a phase shift layer. The defects affect the yield of the masks.
A conventional method for correcting defects in a phase shift mask will be explained with reference to the attached drawings. FIGS. 1a to 1d illustrate sections showing one example of the steps of the conventional method for correcting a defect in a phase shift mask.
Referring to FIG. 1a, a plurality of portions 2 of a light shielding pattern are formed in predetermined intervals on a light transmissive substrate 1, and a resist 3 is deposited on an entire surface of the light shielding pattern. As shown in FIG. 1b, a trench region is defined on the resist 3, and patterned by exposure and development of the resist 3. Then, the light transmissive substrate 1 is etched to a predetermined depth using the patterned resist 3 as a mask, to form a trench 4, which is a phase shift region.
The trench 4 is formed in the light transmissive substrate 1 to alternate between portions 2 of the light shielding pattern. In the formation of the trench 4, a defective region 5 occurs due to residue of a reaction between an etch gas and the light transmissive substrate 1.
As shown in FIG. 1c, an FIB (Focused Ion Beam) 6 is used to selectively remove the defect in the defective region 5. To be sufficient to remove the defect, the energy of the ion beam is large enough so that the beam can also disrupt the crystal lattice of the light transmissive substrate, i.e., one defect is possibly traded for another. As shown in FIG. 1d, the resist 3 is removed to complete fabrication of a phase shift mask that selectively induces opposite phases in the light passing through it.
Another example of the conventional method for correcting defects in a phase shift mask will be explained. FIGS. 2a to 2e illustrate sections showing another example of the steps of the conventional method for correcting a defect in a phase shift mask.
Referring to FIG. 2a, a plurality of portions 11 of a light shielding layer pattern are formed at predetermined intervals on a light transmissive substrate 10 having a dummy region 18 and a main region 19. The light shielding pattern is formed only in the main region 19. Then, trenches 12 are formed in the exposed light transmissive substrate 10 to alternate between the portions 11 of the light shielding pattern. A defective region 13 of non-etched substrate 10 has occurred at one of the trenches 12.
Referring to FIG. 2b, a polymer layer 14, a chrome layer 15 and a resist layer 16 are formed on an entire surface in succession, and the resist layer 16 is patterned to a width wider than the defective region 13 at the trench 12 in the main region 19. The resist layer 16 in the dummy region 18 at a position of a trench formation is patterned in the same way. In this case, the polymer 14 is selected which has the same etch selectivity as the light transmissive substrate 10.
Referring to FIG. 2c, the chrome layer 15 and the polymer layer 14 in the main and dummy regions 19 and 18 are etched (using the patterned resist layer 16 as a mask) until the detective region 13 is exposed. Then, the resist layer 16 is removed.
Referring to FIG. 2d, the defective region 13 in the main region 19 is etched. As the etching speeds are the same, the polymer layer 14 in the dummy region 18 and the defective region 13 are etched at the same speed. That is, the polymer layer 14 in the dummy region 18 as well as the defective region 13 at the trench 12 are etched on the same time until the light transmissive substrate 1 underneath the polymer layer 14 in the dummy region 18 is exposed. And, as shown in FIG. 2e, the polymer layer 14 is removed. In this case, the chrome layer 15 is removed automatically if the etching used in the removal of the polymer layer 14 is a wet etching.
The conventional methods for correcting defects in a phase shift mask discussed above, have the following problems.
First, the use of FIB in removal of the defective region can not remove only the defective region effectively, and causes errors in phase shifting due to the damage from the FIB.
Second, the use of a dummy region in correcting a defect additionally requires formation of the polymer layer, chrome layer and resist layer, as well as processes of alignment, exposure and development. This complicates the process, and reduces the device packing density.
Third, the etching of the substrate underneath the polymer layer in the dummy region has a problem that reworking can be hardly done once the substrate is damaged.